Artificial lift is used in many oil wells for raising oil and water to the surface. One technique which is involved in practice of this invention employs a high pressure multiplex piston pump at the ground surface to provide hydraulic pressure in tubing running down the well. The high pressure power liquid (or power fluid, as it is often called) from the high pressure pump operates a downhole pump for raising the produced fluids to the surface. A variety of such downhole pumps are commercially available and many of them mix the spent power liquid with the produced fluids in the well.
When the mixture of produced fluid and power liquid reaches the ground surface, it is typically separated into water, oil and gas phases by gravity separation. Water or oil from this separation is used as the power liquid.
In large installations where there are a number of wells, the power liquid may be withdrawn from a tank in the storage battery, and cleaned to be suitable for use in the downhole pump. Chemical additives for corrosion inhibition and enhanced lubricity may also be used. Such central stations for power liquid are relatively expensive when the well heads are scattered since appreciable high pressure piping may be required. In many situations it is therefore desirable to provide individual power fluid units for each well. Typically, these are skid mounted units having a separator vessel and high pressure pump to supply high pressure power fluid to a well head and receive and separate the commingled power fluid and production fluid. Such a unit for a single well includes its own prime mover, such as an electric motor or internal combustion engine to drive the pump.
Since the power liquid and production fluid are commingled in the well, particles of solids may become suspended in the power liquid. The efficiency of a downhole pump, particularly a reciprocating piston type, depends at least in part on minimized leakage of power liquid around the piston. To minimize such leakage precision parts are employed with extremely close tolerances between mating surfaces. Very small abrasive particles contained in the power liquid can, therefore, cause damaging wear on the pump parts, requiring replacement and repair of the downhole pump. Unless the power liquid is well cleaned a downhole pump may fail to perform adequately after only a few months of service. It is, therefore, highly desirable to remove particles of abrasive materials from the power liquid down to very small sizes.
Such cleaning can be accomplished in a central station serving several wells but becomes more of a problem in the isolated single well pumping units that are preferred for many oil field operations.
One technique that has been used for cleaning power liquid employs a cyclone centrifugal separator where liquid is introduced tangentially into a conical housing. Relatively dirty liquid containing particulate solids more dense than the liquid is discharged from the small end of the cone. Relatively clean liquid is discharged through the center of the larger end of the cone. Such a single well system is described in U.S. Pat. No. 3,709,292 by Palmour. A cleaning apparatus for power liquid employing cyclone centrifugal separators is also described in U.S. Pat. No. 3,802,501 to Mecusker. A single well unit employing a cyclone centrifugal cleaner has also been available from Kobe, Inc., assignee of this application. Such a unit is described in Bulletin SU-374 from Kobe, Inc. A copy of this bulletin and the aforementioned patents accompany this application.
Cyclone centrifugal separators are sensitive to the underflow flow rate, that is, the proportion of dirty liquid withdrawn from the smaller end of the cyclone. The flow rate needs to be carefully controlled and in a single well power fluid unit, there must be a careful balance of three operating pressures, namely the inlet and two outlets of the cyclone. Although cyclones can work effectively in a controlled situation, it is found in practice during oil field operations, that they often work relatively poorly because of carelessness of operators. Further, the total system, which includes some of the operating parameters of the well, is not a completely stable system and unattended operation over a long period of time can result in changes that adversely affect the operating characteristics of the cyclone. When theh cyclone is not operating properly, relatively large particles can remain in the supposedly clean liquid from the cyclone. Such particles in the power liquid adversely affect the lifetime of pumps.
It is also desirable to provide a pressurized feed to the high pressure multiplex pump employed in a power fluid unit for maximum efficiency. Pressure feed to the high pressure pump may be maintained by keeping the entire separation system and production fluid portion of the well at elevated pressure as described in U.S. Pat. No. 3,709,292. It may also be provided by a circulating pump as described in U.S. Pat. No. 3,802,501.
Thus, it is desirable to provide a single well power fluid system for providing clean power liquid for a downhole pump wherein very small size particulate matter is removed from the power liquid and it is applied to a high pressure pump at appreciable pressure. Such a system should operate with minimal changes in operating characteristics over substantial periods of time without attendance. It is desirable that such a system operate without any appreciable need for sensitive adjustments so that it can be used in oil fields without requiring special operator skills.